A major challenge in the design of full duplex communications systems (i.e., systems which simultaneously transmit and receive) is self jamming. One kind of self jamming occurs when transmit power leaks within the transmit frequency band into the receive path. If the leakage signal power level is sufficiently high, it may degrade the performance of the receive subsystem in one or more ways. For example, it may drive the receive path components into compression and thus reduce sensitivity. Moreover, depending on the receive down-conversion frequency plan, the transmit power or a spur formed in the receive chain may end up in one of the intermediate frequency (IF) bands, directly adding noise and thus reducing receive sensitivity. One approach to guarding against this form of self jamming involves placing a filter at the input to the receive chain prior to the first active component (typically a low noise amplifier, or sometimes a mixer). This filter highly attenuates power in the transmit band whilst providing low loss to signals in the receive band.
Another kind of self jamming involves the leakage of spurious power generated by the transmitter which falls inside the receive band. If this power couples into the receive path, it adds directly to the noise in the receive path, thereby degrading receive sensitivity. One approach to guarding against this form of self jamming involves providing a filter at the output of the transmitter. This filter highly attenuates power in the receive band while providing low loss to signals in the transmit band. The filter must be placed after the last stage of transmit amplification, as this stage is commonly a major contributor to the generation of spurs (as it tends to be operated at or close to saturation to achieve reasonable efficiency).
High isolation filters introduce undesired attenuation in their pass band degrading satellite transmit capability (EIRP) and satellite receive sensitivity (G/T). Techniques utilized to offset these degradations (e.g., higher transmit power, higher antenna gain) increase satellite mass and cost. Moreover, these filters tend to be both heavy and expensive.
Other approaches to guarding against self jamming involve physically separating the transmit and receive hardware to provide spatial isolation. In some satellite systems, the total required isolation at both the transmit and receive frequencies to prevent self jamming is in the range of 100 to 150 dB. This isolation requirement is a system design driver and also drives system cost and mass. In satellite systems providing separate transmit and receive antenna systems, it increases cost and mass (which in turn increases launch cost) because two antennas must be procured and accommodated on the satellite. Designing a satellite to place two antennas far apart (to increase spatial isolation) also increases satellite mass and cost.
Other communications systems which use active phased arrays to achieve improved system flexibility/capability must also guard against self jamming. Phased arrays use a plurality of radiating elements with associated filters, amplifiers and phase/amplitude control devices to form beams whose direction and shape are defined by commanding the phase/amplitude control devices to appropriate phase/amplitude states. The spacing between the radiating elements is determined by the required beam scan. For example, in systems requiring high beam scan (e.g., >45° scan), the elements are typically placed ˜0.5 wavelengths apart. For systems requiring less beam scan (e.g., geostationary communications satellites requiring 5-9° scan) the elements are typically placed ˜2 to 3 wavelengths apart. The combination of this spacing constraint with the large number of radiating elements and associated electronics paths in a phased array limits the isolation level available from practical filters. In some high beam scan systems (e.g., systems using the SHF geostationary satellite communications band: 7.25-7.75 GHz downlink and 7.9-8.4 GHz uplink), the separation between the transmit and receive frequencies is so small that no filter with useful isolation can be fitted within the array radiating element grid. Accordingly, these systems rely entirely on spatial separation to provide the required isolation.